AT APPROX. 2:10 A.M. ON MAY 17, 2004, A GROUND FALL WAS DISCOVERED IN THE 13 NORTH RETURN AIRWAY AT 5 W INTERSECTION. FALL MEASUREMENTS WERE APPROX. 45' WIDE X 70' LONG X 0-8 1/2 FEET THICK. THE FALL AREA WAS TRAVELABLE ALONG THE WEST RIB. MSHA FIELD OFFICE NOTIFIED.